1. Field of the Invention
The present invention relates to a drive and control circuit for high-speed switching of a laser diode in an optical printer and the like, and more particularly to a high-speed drive of a laser diode and stabilization of an optical output from the laser diode.
2. Description of the Prior Art
In what follows, the operation principles of a laser beam printer employed in general will be described with reference to FIG. 14.
Designated at 101 is a video signal generator for generating a video signal to be printed, 102 is a laser diode, 103 is a drive circuit, and 104 is a coupling lens for shaping a light beam emitted from the laser diode 102. Designated at 105 is a scanner motor, and 106 is a polygonal mirror, which is driven by the scanner motor. Likewise, designated at 107 is an f-0 lens, and 108 is a photosensitive drum, the surface of which is scanned with a laser beam transmitted via the polygonal mirror 106 and the f-0 lens 107. Any image so formed on the drum surface is available as a print through a xerography process (not shown).
A laser diode for use in such a laser printer is miniaturized and is capable of being rendered to direct modulation. It is, because of being semiconductor device, critically dependent on a temperature change. Namely, it provides an output remarkably varied depending on temperature. FIG. 2 is a graph illustrating such a characteristic of a laser optical output with respect to temperature when driving the laser diode with a constant current, where the optical output is reduced as the temperature is raised.
There are two factors for such variations in the characteristics of a laser diode due to temperature; a long-term one based on a change in ambient temperature and on the temperature rise of the whole printer apparatus, and a short-term one based on the self-heating of the laser diode.
One of the variations in the characteristics of the laser diode caused by the self-heating is due to the rise of temperature of a casing enclosing the laser diode, which can be resolved by increasing the size of a radiating fin. Another one is due to the temperature rise caused by heat resistance between a drive junction and the casing, which is difficult to be solved from the structural view of the device.
Referring here to FIG. 3 which shows a timing chart for illustrating the dependency of a printed output from the laser printer upon temperature, chip temperature (b) of the laser diode, when the laser diode is powered at a level "H", follows an intermittent printing of data (a) and changes exponentially. In addition, an optical output (c) is distorted with respect to the printing data (a) owing to the starting characteristics of the laser and the temperature characteristics of the laser chip changing in response to temperature as shown in FIG. 2. Print-out data (d) is yielded by assuming a threshold Th of the optical output (c) to be a level indicated by a chain line in the same figure, in a xerography printing process. Such temperature characteristics will be examined together, hereinafter, with the starting characteristics of the laser diode.
The aforementioned print-out data (d) is shifted with respect to the printing data (a) particularly owing to the temperature characteristics of the optical output (c) at the initial power-on time of the laser diode.
FIG. 4 (A) shows an original picture image corresponding to the printing data (a) of FIG. 3, and FIG. 4 (B) shows that a picture image restored from the print-out data (d) of FIG. 3 is distorted. Namely, the situation in FIG. 4(B) is yielded as in FIG. 3 by scanning the figure from the left to the right, and when the laser diode is deenergized over positions Y1 and Y2 for a short period of time and thereafter energized as shown by the scanning over a region X1, the light-off time corresponds to a time interval t1-t2 of FIG. 3, and an off-position Y1'(corresponding to time t1) substantially accords with the position Y1 of the printing data but a successive on-position Y2'(corresponding to time t2) is delayed by a time fraction T1 from the position Y2 of the printing data. In addition, when the laser diode is deenergized over a long period of time corresponding to a time interval t3-t4 as in the scanning over a region X2 and then energized, the on-position Y4' is delayed by a time fraction T2 with respect to the position Y4 of the printing data. Hereupon, inequality T1&gt;T2 holds because of the steep rise of the optical output (c) after the laser is deenergized over a long period of time, and these time durations T1 and T2 are minute so that the change of the printing in full size is negligible. However, the position Y2' is delayed by T11.varies.T2 from the position Y4', and hence the printing is displaced to result in the deterioration of printing quality.
The frequency response of the laser diode including the starting characteristics thereof will be described herein.
In the optical printer, the maximum switching frequency f of the laser diode is expressed by f=c LX LY D.sup.2 P where LX is the lateral size of a printing paper, LY is the longitudinal size of the same, D is dot density (lines/mm), P is a printing speed (sheets/min), and C is a constant. However, with an optical printer requiring higher speed and density in recent years, a high-speed switching drive circuit of a laser diode is also needed . In addition, the waveform of a drive current of a laser diode is desired to be substantially the same as that of an input pulse as an image signal from a viewpoint of printing quality. Accordingly, the drive circuit, since the drive current waveform is a square wave, must have a frequency response sufficiently higher than the aforementioned maximum switching frequency f. Furthermore, the drive waveform is desired to be stable at a high speed since no overshooting and ringing are preferred from the viewpoint of printing quality upon the building-up and building-down of such a waveform.
FIGS. 15 and 16 show such prior switching circuits. The circuit shown in FIG. 15 is adapted to have a laser diode 1 connected to a constant-current source 3 via a transistor 2 and a series circuit of a transistor 4 and a resistor 5 connected in parallel to a series circuit of the laser diode 1 and the transistor 2, the transistor 4 being switched on upon receiving an L level signal of a video signal Vin on a base thereof while being switched off upon receiving an H level signal.
The transistor 2 is, while receiving a plus potential V2 on the base thereof, set to permit the laser diode 1 to be deenergized as the transistor 4 is switched on, and to provide impedance reduction thereof required for laser eneragization as the transistor 4 is switched off. The switching circuit shown in FIG. 16 in a simplified form of the circuit of FIG. 15 is adapted to permit the laser diode 1 to be energized by allowing the transistor 6 to be switched off upon the video signal Vin being the L level, while it is adapted to permit the laser diode 1 to be deenergized owing to a voltage drop thereacross by allowing the transistor 6 to be switched on upon the video signal Vin changing to the H level.
Although it should force the transistor 2 to alter voltage on the emitter thereof at a high speed, the circuit of FIG. 15 is limited in its operation by the frequency characteristics of the constant-current source 3. Likewise, the circuit of FIG. 16 suffers from the problem that the laser diode 1 depends, in its transition from its emitting to non-emitting state, on the impedance characteristic in itself, but the characteristic is unstable because the impedance of the laser diode 1 is sharply changed substantially from infinity to several tens of ohms around a threshold current thereof together with the impedance characteristic being altered owing to the capacitance thereof.
Let us here consider an optical output in a beam scanning device such as an optical printer and the like.
A control circuit for such an optical output is adapted, for example when scanning a drum surface of the printer with an optical beam through a rotating polygonal mirror, to set an optical output level of the laser diode in successive printing scanning in synchronization with a beam detection signal produced for each period not corresponding to a picture image, during which the polygonal mirror alters finishing, printing and scanning of one line and begins to repeat the same operation for the next line.
There are known some conventional methods to set such an optical output level. For example, one method is adapted to compare, in a period not corresponding to a picture image, an optical output level signal detected by an optical detector with a reference level in an analog manner, and sample-hold the result compared as such. Another method in a digital system is adapted to count pulses within a prescribed period or to stop the count operation in response to the positive or negative sign of such a compared signal, and to D-A convert the resulting counted valve.
However, with progress of such optical printers speeded up in their operation, control time for an optical output needs to be reduced more and more. Those devices as described above encounter thereupon the severe problems that they can not sufficiently deal with changes of the characteristics of a laser diode due to temperature rise thereof and the like since they are, particularly in an analog system, incapable of control for each line of printing because of their slow response to the comparison control and hence are obliged to effect the control for each page of printing. Those devices in a digital system furthermore suffer from problems other than the response speed described above. For example, variations of an optical output in a laser diode chip are insufficiently compensated and the number of bits of a D/A converter is increased, provided the amount of light to be controlled in one step of a counter is reduced for improving control accuracy.